Suppose that in 5 years or so I send you a Yelp review of a restaurant, from my phone to yours. What will that mean?

First, I might well use something like Airdrop, or touch my phone to yours to pass it across, or tell Now to give it to you, or indeed Now might decide to give it to you without my even explicitly asking. Or the review might be invoked by a Bluetooth LE beacon as you hold your phone next to the menu on display by the door

But for the sake of simplicity, suppose I send it to you using an internet messaging app - either one built into the OS or a third-party one - Facebook, Whatsapp or more probably one that doesn't exist yet but by then has 15 engineers and 1bn MAUs.

It seems pretty unlikely that you'll see a dumb URL string on your screen. Rather, you'll get something rich and interactive, within the message.

And you'll be able to go into that experience and tap the number to call the restaurant, or make a live booking, or swipe through photos.

And if you tap 'book', it'll pass them a $10 booking fee in bitcoin, authorised with a fingerprint swipe.

Now suppose you decide to save this item, as an icon on your home screen, or some other yet-to-be created place.

Now, what were you using? An app? a widget? Native code? What programming language? Did you install an app or surf the web? I'd suggest that none of those questions would really mean anything, at least not as we think of them right now. The programming language matters much less than the user flow. And some of this example sounds 'webby', but Google is the first to advance interaction models that are not remotely webby (such as Now).

This is a pretty simple illustration (an expansion of the super-hot card metaphor) of a broader point I've made before: on the desktop internet, the web was by far the dominant model and that didn't actually change very much for well over a decade (before that, the interfaces of Windows and Mac were also very stable for a long time). But on mobile, not only are other models just as important as the web, but they're not remotely stable, settled or mature. The platform war may be over but that doesn't mean things are settling down.

So I have very little idea what precisely I would mean if, in 5 years, I were to say 'I installed an app on my smartphone'. Further, I'm pretty sure that if it's an Apple smartphone it will run an iteration of iOS but I'm rather less sure what Google will have done with Android and Chrome by then. And of course I might be running a fork of Android from Amazon or, perhaps, Microsoft.

This is the key reason why the new social messaging apps are so interesting - not because they have users and inventory now, but because they can be vectors for some of this sort of behaviour - a third acquisition, discovery and distribution channel besides Facebook and Google.

This may also have implications for any discussion about what it means for Apple that its ecosystem will have a minority of mobile users. We need to think about what it means to call a ecosystem that might have 800m-900m live devices 'minority', but we also need to think about what 'ecosystem' might mean. What, if any, 'winner takes all' dynamics operate in this environment? One reason the Mac didn't die was because the web changed what it mean to be a computer ecosystem: the mobile ecosystem has lots of changes to come too.

I work at Andreessen Horowitz ('a16z'), a venture capital firm in Silicon Valley that invests in software companies. I try to work out what's going on and what will happen next.

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